A critical component of computer systems is data storage. Data storage can be divided conceptually into an individual user's data storage, which is attached directly to the individual's computer, and network based data storage typically intended for multiple users.
One type of network based storage device is a disk array. The disk array includes at least one controller coupled to an array of disks. Typically, components (e.g., the controller and the disks) of the disk array are hot swappable, which allows components to be replaced without turning off the disk array.
As an alternative to the disk array, researchers have been exploring data storage within a distributed storage system which includes an array of independent storage devices coupled together by a network. Each of the independent storage devices includes a processor, memory, and one or more disks. An advantage of the array of independent storage devices is lower cost. The lower cost can result from mass production of the independent storage devices as commodity items and from elimination of hot swappable features of the disk array. Another advantage is better scalability. The user can buy a few devices initially and add more devices as demand grows.
A separate development in the field of data storage is mirroring of data between a local site and a remote site, which may improve data accessibility. Existing synchronous mirroring schemes use some form of master-slave or primary-secondary replication to achieve data consistency and input/output ordering. These techniques use a single master (i.e., a single controller, a single in-band network switch, or a single virtualization appliance) that orders all input/output requests and directs them to each side of a mirror (e.g., a local site and a remote site). This single point of control is a performance bottleneck that may reduce accessibility. It would be advantageous to be able to use synchronous mirroring for the distributed storage system but such systems lack a single point of control making existing synchronous mirroring techniques not feasible for the distributed storage system.